This invention relates to an infrared ray gas analyzing apparatus in which the infrared ray wavelength absorption characteristics inherent in various gases are utilized for measuring the amount of energy absorbed, to thereby subject a sample gas to quantitative analysis.
In general, an infrared ray gas analyzer is extensively used for analyzing various gases because it is excellent in stability and reliability. For instance, it is used for detecting a minute quantity gas contained in the air, or the low density CO gas in the air for the purpose of circumstance security, pollution prevention and pollution monitoring.
FIG. 1 is a schematic diagram showing a conventional infrared ray analyzer for measuring CO gas. A sample gas, for instance the air, is distributed from a sampling conduit 1 through a branch pipe 2 to a reference gas conduit 3 and to a sample gas conduit 4. The reference conduit 3 is provided with a converter 5 and a flow meter 6, and is connected to a reference cell 13 in an infrared ray analyzer 8. Accordingly, the gas in the conduit 3 flows into the reference cell 13 and flows out of the latter through a conduit 16 connected to the reference cell 13. The converter 5 is a small combustion furnace where CO gas is burnt at a lower temperature with the aid of a catalyst so that the CO gas is converted into CO.sub.2 gas. The flow meter 6 is provided with a flow rate control valve for supplying a certain quantity of reference gas to the analyzer 8. On the other hand, the sample gas conduit 4 is provided with a flow meter 7. After a certain quantity of sample gas is supplied to a sample cell 14 in the analyzer 8, it is discharged through an exhaust pipe 17 connected to the cell 14.
A measurement light beam emitted by an infrared ray source 9 is subjected to absorption in correspondence to the density of the analysis gas CO in the sample gas in the sample cell 14. On the other hand, a reference light beam emitted by an infrared ray source 10 is passed through the reference cell 13 without being absorbed, because there is no CO gas in the reference gas. The measurement light beam passed through the sample cell 14 and the reference light beam passed through the reference cell 13 are alternately or simultaneously interrupted by a chopper 12 which is rotated at a constant speed by a chopper motor 11, and a detector 15 detects the difference in light quantity between the reference light beam and the measurement light beam as an electrical pulsating current, which is rectified and amplified. The current thus treated is outputted by the detector 15. Thus, the difference in light quantity corresponds to the density of the analysis gas in the sample gas.
In an infrared ray gas analyzer of this type, the decrease in light quantity of the measurement light beam with respect to the light quantity of the reference light beam is provided as its output. Therefore, the detection sensitivity in analyzing a low density gas is insufficient and, furthermore, it is difficult to increase the detection sensitivity. Furthermore, the conventional infrared ray gas analyzer is disadvantageous in the following points. The mist (polluting material) in the sample gas is deposited on the inside surface of the sample cell 14, which lowers the reflection and transmittance conditions. The gas created when the sample gas is subjected to reaction through the converter 5 pollutes, in the form of mist, the reference cell 13, which also leads to variations of the reflection conditions. As a result, the optical balance is lost, which may cause zero point drift.
Shown in FIG. 2 is another infrared ray gas analyzer, in which those components having the same functions as those in FIG. 1 are designated by the same reference characters. A sample gas conduit 18 and a reference gas conduit 19 are switched at predetermined time intervals by a three-way valve 20 driven by a valve driving device 21, to thereby be connected alternately to a gas conduit 22. As a result, the sample gas or the reference gas 19 is introduced into the sample cell 14 in the analyzer 8 and is discharged through the discharge pipe 17. A gas, for instance N.sub.2 gas, which does not absorb infrared rays is sealed in the reference cell 13. When the sample gas conduit 18 is connected to the gas conduit 22 by operating the three-way valve 20, the measurement light beam emitted by the infrared ray source 9 is subjected to infrared ray absorption in correspondence to the density of the analysis gas in the sample gas in the sample cell 14; however, the reference light beam emitted by the infrared ray source 10 is not subjected to infrared ray absorption in the reference cell 13. Accordingly, the difference in light quantity between the measurement light beam passed through the sample cell 14 and the reference light beam passed through the reference bath 13 is detected by the detector 15, the output of which corresponds to the density of the analysis gas. When the output of the detector becomes stable, a switch SW1 is closed. As a result, the output of the detector amplified by means of a rectifier 23 and an amplifier 24 is stored and held in an output holding circuit H1. Then, in a predetermined period of time shorter than several minutes, the reference gas conduit 19 is connected to the gas circuit 22 by operating the three-way valve 20, as a result of which the reference gas (such as N.sub.2 gas) is allowed to flow in the sample cell 14. In this case, the measurement light beam is not absorbed. Accordingly, the light quantity of the measurement light beam passed through the sample cell 14 is equal to that of the reference light beam passed through the reference cell 13, and the difference in light quantity detected by the detector 14 is zero. However, similarly as in the firstly mentioned conventional infrared ray gas analyzer, the inside surface of the sample cell 14 is contaminated by the polluting material in the sample gas, as a result of which the optical balance between the sample cell and the reference cell is lost, which may lead to the occurrence of the zero point drift. In order to eliminate the effect of this zero point drift, when under this condition the output of the detector 15 becomes stable, a switch SW2 is closed. As a result, the output of the detector is stored and held by an output holding circuit H2. Thus, a differential amplifier 25 obtains the difference between the outputs of the output holding circuits H1 and H2, thereby to output a measurement value (measurement signal). Therefore, the zero point drift caused by the optical unbalance between the sample cell 14 and the reference cell 13 is automatically corrected. Nevertheless, the conventional infrared ray gas analyzer is also disadvantageous in that, as the pollution of the sample cell 14 is increased thereby to cause a great optical unbalance on one side, the amount of correction of the zero point drift is increased. Furthermore, the detection sensitivity is still insufficient, similarly as in the analyzer shown in FIG. 1.